Our research seeks to understand how chromatin regulatory mechanisms influence nuclear and epigenetic programs, and how de-regulation of these mechanisms contributes to aging and disease. SIRT6 is a chromatin regulatory factor in the sirtuin family of enzymes. SIRT6-deficiency in mice leads to shortened lifespan and phenotypes associated with aging, cancer, and metabolism. Conversely, SIRT6 over-expression in mice protects against metabolic disease and extends lifespan. Thus, studying SIRT6 function promises to elucidate fundamental mechanisms that underlie healthy aging and longevity. Previously, we showed that SIRT6 selectively regulates specific chromatin marks associated with epigenetic and gene-regulatory functions. We linked chromatin regulation by SIRT6 to key nuclear processes that impact on aging and cancer, including telomere maintenance, DNA repair, and aging-associated gene expression changes. Here, we focus on new functions of SIRT6 in chromatin silencing mechanisms that are deregulated in aging. We propose molecular, genomic, and functional studies to study the role of SIRT6 in maintaining heterochromatin silencing at repetitive DNA elements, and ask how impaired silencing leads to cellular dysfunction associated with aging. In Aim 1, we will study the molecular mechanisms of SIRT6 in heterochromatin silencing of repetitive satellite DNA elements at centromeres. Defects in centromeric heterochromatin are observed in the contexts of both aging and cancer. We will characterize the biochemical activity of SIRT6 at centromeric chromatin, how this is regulated during mitosis, and how it affects higher order chromatin changes. Our studies will provide insights for cancer cell biology, where SIRT6 loss may contribute to cancer progression, and for human somatic cells, where SIRT6 may guard against cellular senescence or age-dependent decline in epigenetic plasticity. In Aim 2, we will characterize the functional effects of heterochromatin maintenance by SIRT6 on cellular homeostasis. We will test the hypotheses that heterochromatin loss at centromeres triggers abnormal mitoses, chromosome segregation defects, and cellular senescence, and can facilitate the oncogenic process of cellular immortalization. We will also examine functional interplay between SIRT6 and other SIRT enzymes in these processes. These studies should elucidate how heterochromatin breakdown is translated into cellular phenotypes or functional decline that contributes to aging and disease. In Aim 3, we will investigate the role of SIRT6 in heterochromatin maintenance at another class of repetitive DNA elements that are deregulated in aging and cancer - endogenous retrotransposable elements. We will ask if impaired silencing of these elements leads to genomic instability that can affect cellular function, or to aberrant transcription of aging- relatd genes. Together, these studies should provide insights into fundamental chromatin mechanisms in aging biology.